From WO 02/30638 a polymerizable solution is known containing at least furfuryl alcohol and an acid compound selected from maleic anhydride, phthalic anhydride, maleic acid, malic acid, phthalic acid and a combination thereof. The solution is used to impregnate wood and the impregnated wood thus obtained is subjected to a curing step, involving the heating of the impregnated wood at a temperature of 70 to 140° C. The result is impregnated wooden article with an even dark colour.
U.S. Pat. No. 6,747,076 describes a two-part composition comprising furfuryl alcohol and a by-product of a chemical pulping process as a first component and a maleic anhydride-containing catalyst composition as second component. The by-product is a lignin-comprising polymer mixture. The two-part composition according to U.S. Pat. No. 6,747,076 is particularly suitable as wood adhesive. In that context a wooden surface to be bonded is coated with the composition and brought into contact with another wooden surface which may or may not have been coated with a similar or the same coating, and the contacting surfaces are allowed to bond under the influence of heat and pressure.
Lignin is a complex polymer of aromatic alcohols. It is commonly derived from wood and can be found in cell walls of plants. It is known as an adhesive in its natural state. Since dissolved lignin has some reactive groups that allow it to react with adhesive formulations, lignin has been employed in various wood adhesive formulations. Usually the lignin is employed as an extender in adhesives based phenol-formaldehyde resins and the like. Lignin is a cross-linked macromolecule which is relatively hydrophobic and aromatic in nature. The molecule consists of various types of substructures. The macromolecule is synthesized from various monomers, including p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. The distribution of the monomers in lignin is dependent on plant species and tissues. However, typical structures for lignin are phenyl groups, hydroxyl and methoxy moieties and ether bonds.
Typical wood pulping processes include the sulphite, kraft and soda processes. In the latter processes wood is contacted with a sodium hydroxide and some other chemicals to yield lignin. When wood pulping is carried out using the sulphite process, ligno-sulphonates are formed. In US 2006/0292366 a formulation for impregnating wood has been described wherein in addition to furfuryl alcohol and an acidic initiator, a water-soluble stabilizer, such as the calcium or ammonium salt of a lignosulphonic acid can be used. The most reactive lignins can be isolated from an organosolv process, in which wood is subjected to pulping using organic solvents such as ethanol and/or acetone. An organosolv process is described in e.g. US 2013/172628.
U.S. Pat. No. 3,293,200 describes thermosetting adhesive compositions useful in the manufacture of plywood and containing, as essential ingredients, a water-soluble phenol-aldehyde resin and a water-insoluble, finely-divided humin material obtained from the manufacture of levulinic acid by acid hydrolysis of lignocellulose, i.e. a combination of carbohydrates and lignin. Although various procedures are known for the manufacture of levulinic acid from lignocellulose, the reaction generally is carried out under severe acid hydrolysis conditions at a temperature in excess of 150° C., usually between 170° C. and 210° C. in the presence of a strong acid catalyst. During the reaction, the humin material is formed as a result of an acid breakdown of lignocellulose while substantially the entire carbohydrate portion of the lignocellulose is degraded and the hexosans converted to the desired levulinic acid. According to U.S. Pat. No. 3,293,200 humins are the water-insoluble solid residue, containing predominantly lignin compounds. Furthermore, U.S. Pat. No. 3,293,200 states that residues obtained from acid hydrolysis of pentosan-containing lignocellulose under mild conditions to yield furfural are not contemplated in the practice of this invention. Such materials contain a substantial proportion of the original unreacted carbohydrates which remain in the residues together with the lignin.
It has been found that the formulations comprising furfuryl alcohol and an acidic initiator and formulations comprising these components together with lignin fail to provide optimal results as wood adhesive or as impregnating agent for wood and other materials. Surprisingly, it was found that the addition of humins to furfuryl alcohol and acidic initiator provided improved results over the use of furfuryl alcohol and acidic initiator alone.
In this specification humins are the coloured bodies which are believed to be polymers containing moieties from hydroxymethylfurfural, furfural, carbohydrate and levulinic acid. These coloured bodies are i.a. produced as by-products in the partial degrading of carbohydrates by heat or other processing conditions, as described in e.g. EP 338151. The use of humins has been described in DE 3621517, where it is stated that the humins, i.e. the by-product of the preparation of alkoxymethylfurfural and alkyl levulinates from cellulose, lignocellulose or starch with an alcohol, can only be used for the provision of heat by combusting it. It would therefore be very advantageous, if humins can be used in a more environmentally-friendly and value-added application.